The present invention relates to hardening protection compositions for partial carburization of metallic components.
In the heat treatment process for surface hardening of metals, it is often necessary for components to be partly protected before the treatment process, such as carburization, nitrocarburization or nitriding, in order still to be able to carry out certain mechanical working steps later on in selected surface areas, or in order to be able to retain the original properties of the material, such as, for example, the ductility.
In addition to electroplating with copper or nickel, for a long time there has already been a wide range of lacquer-like protective compositions which are applied to the selected surface areas before carrying out the hardening treatment. In the case of partial carburization, protective composition based on water-glass or based on substances which form boron glass have proved themselves useful for protection against diffusing-in of carbon.
While the compositions based on water-glass can be purged only mechanically after the hardening operation by blasting the components with sand or glass beads etc., protective compositions based on substances which form boron glass have the great advantage of being able to be washed off with water. However, with the known hardening protection compositions based on substances which form boron glass, there is the risk of running off in the oven during the hardening operation, in particular after incomplete drying or due to binding of moisture from the atmosphere by the composition, since the viscosity of the boron components is greatly reduced by water at a high temperature.
Furthermore, at carburization temperatures of 900–980° C. the boron compound can vaporize until vapour pressure equilibrium is established. On the one hand this results in a decrease in the protective action due to the protective layer becoming thinner, and on the other hand an oven lining of SiO2-containing bricks can also be attacked. In particular, because of the relatively high vapour pressure such compositions can be employed to only a limited extent in reduced pressure carburization, since damage to the reduced pressure carburization plant by vaporizing boron compounds must be expected.
The protective compositions based on water-glass hitherto employed as an alternative in vacuum carburization plants tend to become brittle and flake off during high-pressure gas quenching. The treatment plants then become contaminated. The heat exchangers can thus become covered with particles, or the fan bearing can be damaged, which can lead to a plant shut-down.
Therefore, an object of the present invention was to discover and develop hardening protection compositions for partial carburization of metallic components which do not have the disadvantages of the known products.